5 of the 10 deadliest wars were in a China. Most of them were before the booms. Why was it that such high levels of their populations used to die on the wars? Most wars in Europe had average death tolls of at most 3 million. But why did China have death tolls like 30 million, 20 million and so on?
Because most likely not all of them died. Historians calculate death toll by comparing census figures from before and after the rebellions, but census data is problematic because it only takes into account registered households. People displaced by fighting are not counted, so you end up getting a difference of tens of millions of people before and after the rebellion. Rather than treat these figures as number of people dead, you should instead treat it as number of people affected, this would include both the dead and the displaced. You should also consider the impact of starvation and diseases. For example, one historian estimates that only 30% of people died as a result of fighting during the Taiping Rebellion, while the rest died of cholera (and probably starvation as well).
With regard to the Taiping in particular, I point you towards my old answer here.